A two-phase random medium is generally a domain of space \(\mathcal {V}\subseteq \mathbb {R}^d\)  that is partitioned into two disjoint regions: a region of phase 1, \(\mathcal {V}_1\) , and volume fraction \(\phi _1\)  as well as a region of phase 2, \(\mathcal {V}_2\) , of volume fraction \(\phi _2\)  [1]}. The phase indicator function \(\mathcal {I}^{(i)}(\mathbf {x})\)  for a two-phase medium is defined as
\(\mathcal {I}^{(i)}(\mathbf {x}) = {\left\lbrace \begin{array}{ll}1, & \mathbf {x} \in \mathcal {V}_i,\\0, & \mathbf {x}_i \notin \mathcal {V}_i.\end{array}\right.}\) 
